Dr. Stephanie Fulton of the University of Montreal's Faculty of Medicine and colleagues fed one group of mice a low-fat diet, which was 11 percent fat, and a high-fat diet, which was 58 percent fat, to a second group over six weeks, monitoring how the different food affected the way the animals behaved.

The high-fat diet caused the waist size in the latter group to increase by 11 percent, but the mice were not yet obese. The research team used a variety of techniques to evaluate the relationship between rewarding mice with food and their resulting behavior and emotions. The researchers then looked at the brains of the mice and found them physically altered.

One of the molecules in the brain examined was dopamine, which enables the brain to reward with good feelings -- encouraging people to learn certain kinds of behavior, Fulton said.

This chemical is the same in humans as it is in mice and other animals.

In turn, CREB is a molecule that controls the activation of genes involved in the functioning of our brains, including those that cause the production of dopamine and it contributes to memory formation.

"CREB was much more activated in the brains of higher-fat diet mice and these mice also have higher levels of corticosterone, a hormone that is associated with stress. This explains both the depression and the negative behavior cycle," Fulton said in a statement. "A change of diet then causes withdrawal symptoms and a greater sensitivity to stressful situations, launching a vicious cycle of poor eating."

The findings were published online in the International Journal of Obesity.

United Press International is a leading provider of news, photos and information to millions of readers around the globe via UPI.com and its licensing services.

With a history of reliable reporting dating back to 1907, today’s UPI is a credible source for the most important stories of the day, continually updated - a one-stop site for U.S. and world news, as well as entertainment, trends, science, health and stunning photography. UPI also provides insightful reports on key topics of geopolitical importance, including energy and security.

A Spanish version of the site reaches millions of readers in Latin America and beyond.

UPI was founded in 1907 by E.W. Scripps as the United Press (UP). It became known as UPI when after a merger with the International News Service in 1958, which was founded in 1909 by William Randolph Hearst. Today, UPI is owned by News World Communications.